The level of the signal is commonly measured by means of a measurement channel comprising an input amplifier followed by a detector module which is itself followed by an integrator. The amplifier (which includes bias means) produces an output signal comprising both an amplified version of the input signal and a DC component resulting from the bias and from various threshold voltages that stem from the conduction characteristics of the components used. The detector module which receives this signal at its input applies full- or half-wave rectification to the fraction of the signal which is proportional to the input signal, retains said DC component, and adds a voltage offset thereto due to its own bias means and threshold voltages. The integrator disposed at the output of the detector module thus produces an integration signal which is the sum of a voltage proportional to the level of the input signal and an offset voltage due to various bias voltages and threshold voltages.
In order to eliminate the offset voltage, a differential measurement amplifier is generally used receiving both the integration signal and a correction signal, and producing a measurement signal which is the difference between its two input signals. The correction signal must be equal to the offset voltage. It is commonly provided by means of a potentiometer connected to the power supply terminals of the circuit. This potentiometer is then adjusted for a given power supply voltage and a given temperature so that the correction signal is equal to the offset signal, e.g. when there is no input signal. If the power supply voltage and/or the temperature of the circuit vary, then the correction signal and the offset voltage vary in different manners and as a result their difference is no longer zero. The circuits of the measurement channel include semiconductor components which are non-linear and which have temperature coefficients that are very different from the temperature coefficient of a potentiometer. This gives rise to an error which is not acceptable in some applications.
An object of the present invention is thus to provide a circuit for measuring the level of a signal and in which the offset voltage and the correction signal have the same value regardless of operating conditions.